These is my own thoughts submitted to the Choreography working group.
We are interested in feedback, suggestions and the like so that we can:
1. Better understand the issues
2. Capture realistic requirements
3. Work with other groups to achieve 1 and 2
I would appreciate a reply cc'd to
public-ws-chor@w3.org
Cheers
Steve T
co-chair W3C Web Services Choreography
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What can semantics be used for in a choreography description language?
To explore this question we need to define a few basic concepts and
terms.
I make the distinction between locating a service and a choreography. A
service is a Web Service
that exhibits a WSDL interface. A choreography is not a Web Service
but a contract between two or more
services.
In order for a service to be part of a choreography it must agree to
abide by the contract and so exhibit the
required external observable behaviour.
Choreographies are non-executable descriptions of observable behaviour.
They are loosely bound to the
services that they describe which allows services to join and leave a
choreography dynamically. A description
of a choreography fully elaborates the externally observable message
exchanges between a collection of
services.
Consider a service P that sends a message to service Q and waits for a
response while Q sends a message
to service R and then continues doing something else but is now able to
receive a response from R asynchronously,
which is required for Q to respond to P. This is exactly the observable
behaviour that a CDL should describe. It includesl
all of the dependencies that are externally visible. In this case the
response from R back to Q is a dependency for
P. So a CDL will describe the visible "interactions" that occur and
their visible dependencies.
A choreography may include certain system level interaction that are
also externally observable. To a large extent
the external observable behaviour as at the level of a business model
as opposed to a system model [aka MDA].
System model behaviour that is observable might be said to be
transaction boundaries in either a conventional ACID based
approaches or long running transactions of some flavour. System model
behaviour might include any observable exceptions
that are propagated by the participant services.
Service location/selection
Semantics in a CDL are most likely to be used in support of a query
that may be used to dynamically locate a choreography by a Web Service.
This is akin to a conversation (that may be in play already) in which
someone enters a room listens for a while and decides they wish to
participate
or not - in which case they might move to another room and listen in on
another conversation and so on. The sort of thing we do at parties.
This sort of conversation surfing that we do at parties is a good
analogy for the way in which web services might come together to
accomplish
some user-driven goal. They also surf for conversations that we call
choreography descriptions. They evaluate the conversation, not by
listening,
but by engaging in a short chat to identify if the conversation makes
sense for them (i.e. do they understand it at all), that they are not
going to get into trouble by entering (i.e. It all sounds too political
for me to enter into this conversation) or that they do understand,
feel comfortable with but decide that one of the individuals they don't
like and so move on.
For a Web Service it might look more like the following:
I am a Web Service called P.
I would like to participate in a choreography to sell paint? (do I
understand what they are talking about)
I would like to participate in a choreography to sell paint such that
the choreography is lockfree? (am I comfortable)
I would like to participate in a choreography to sell paint such that
the choreography supports a
role called "paint mixer"? (do I have a way into this conversation)
I would like to participate in a choreography to sell paint such that
the choreography supports the an exchange
between a paint broker role and a paint shipper role where the broker
and shipper never directly communicate
to me as a supplier? (I'll only join in if I don't have to talk to ...)
I would like to participate in a choreography to sell paint such that
the choreography supports a transaction model
that is the same as mine? (can I steer it into my comfort zone)
I would like to participate in a choreography to sell paint such that
the choreography but I need to be sure that it understands
the concept of "deferred payment"? (I understand the overall
conversation but I never heard of that word before)
The bracketed part at the end of each query tries to re-phrase what is
being asked in party terms.
To be able to support such queries requires that a choreography makes
visible the semantics of liveness, roles and allows information about
roles and behaviour to be inferred.
The query about transaction models might require the use of clear
separation into system and conceptual artifacts of a choreography. It
might also require some form of behavioural equivalence such a
bi-simulation over all of part of the choreography description.
The last query, about deferred payment, might be a combination of
behavioural equivalence and some form of ontology matching. The
ontology matching might be used to find an equivalence relationship
between "deferred payment" and "late payment".
I am not claiming that we can achieve all of this. What I am trying to
do is put some meat on what semantics might mean to a choreography
description language. I do like the idea of Web Services locating a
choroegraphies with which they can be shown to be compatible and then
joining those choreographies in the right way.
To do the inferencing example we would be well served if we utilised
RDF or something similar. The use of concept matching, "deferred
payment" vs "late payment", would probably necessitate the use of
ontologies (and so OWL or OWL-light) to express the ontologies and the
choreography would make some reference to the ontology that it uses
which could then enrich the choreography description with the
appropriate concepts.
Summary
So to summarise I think that there are two areas in which semantics
play a role in choreography. Firstly to locate a choreography and check
it matches the requirements of a requesting Web Service. Secondly to
reuse ontologies and the concepts that they describe as terms in a
choreography description linking the description to the choreography.
Phew! That is as much as I can write on this topic without making it
totally unreadable, which it probably is anyway.
Cheers
Steve T
p.s. As a small anecdote on the importance of semantics and how easy it
is to get it wrong:
My father worked for the GEC in the UK for many years. He was
responsible for QA (both hardware and software). He told me at the
weekend
that he has asked his PA to draft a letter to be sent to some
sub-contracting company that GEC were using at that time. In dictation
he said
something like:
"I would like to see your QA procedures so that I can assess the
effectiveness of your QA system."
Alas in production the letter sent turned out to be:
"I would like to see your QA procedures so that I can assess the
effectivemess of your QA system."
As you can see even a single letter can change the meaning of things.